The present invention relates to the art of selective nerve stimulation. The invention finds particular application in conjunction with urination control and will be described with particular reference thereto. It is to be appreciated that the invention is also applicable to control systems for fecal incontinence, penile erection, and others.
The organs involved in bladder, bowel, and sexual function receive much of their control via the second, third, and fourth sacral nerves (S.sub.2, S.sub.3, and S.sub.4). While one level of roots usually predominates for a particular function, there is considerable overlap. For example, the S.sub.3 sacral nerve is the main stimulus for both bladder and rectal wall contraction. Bladder and rectal wall both receive some control also from S.sub.4 and/or S.sub.2 sacral nerves. Sphincters are probably mainly innervated by S.sub.4, although the urethral sphincter has significant contributions from S.sub.3. Hence, there is difficulty in applying artificial stimulus to contract the bladder without contracting the urethral sphincter and to contract the rectum without contracting the anal sphincter.
The external urethral sphincter receives stimulation on the sacral ventral roots to cause contraction to block urine flow. To discharge the bladder, in a healthy person, the bladder detrusor muscles are contracted to expel urine simultaneously with relaxing the urethral sphincter to allow the passage of the urine. The contraction of the bladder is also controlled by the sacral ventral roots. More specifically, contraction of the bladder detrusor muscles is caused by smaller diameter S.sub.3 nerves and contraction of the urethral sphincter is controlled by predominantly larger diameter S.sub.3 nerves as well as by S.sub.2 and S.sub.4 nerves which are intermixed in same roots.
Previously, electrical stimulation has been applied to control the bladder and bowel. The previous attempts have focused on three techniques: direct stimulation of the detrusor muscle, activation of the detrusor by stimulation of the conus medullaris, and activation of the detrusor by sacral root or nerve stimulation with extensive dorsal rhizotomy. All three of these methods suffer from the same problem. They all cause contraction of the bladder to expel urine concurrently with contraction of the external urethral sphincter blocking urine flow. The rhizotomy technique also results in the loss of erection for the male. It would be advantageous if contraction of the sphincter could be selectively blocked.
Techniques that are available for blocking nerve impulses are discussed, for example, in "A Technique for Collision Block of Peripheral Nerve: Single Stimulation Analysis", van den Honert and Mortimer, IEEE Transactions on Biomedical Engineering, Vol. BME-28, No. 5, May 1981, pages 373-378 and "Generation of Unidirectionally Propagated Action Potentials in a Peripheral Nerve by Brief Stimuli", van den Honert and Mortimer, Science, Vol. 206, December 1979, pages 1311-1312. With the van den Honert and Mortimer techniques, a nerve impulse or action potential is generated Which travels toward the brain. When the artificially generated nerve impulse meets a motor impulse travelling from the brain, the motor impulse is collision blocked. That is, the artificially generated action potential cancels the motor action potential. If one were to apply the van den Honert and Mortimer techniques, it could be used to cause concurrent relaxation of both the bladder contracting muscles and the urethral sphincter. Again, the bladder contracting muscles and the urethral sphincter are operating at cross purposes.
A technique for fatigue resistant flexing of muscles of laboratory animals, particularly feline hind leg muscles, is described in "A Method for Attaining Natural Recruitment Order in Artificially Activated Muscles", Zi-Ping Fang and J. Thomas Mortimer, IEEE Ninth Annual Conference of the Engineering in Medicine and Biology Society, 1987. By way of background, applying an electrical potential (or current) of an appropriate amplitude along the length of a nerve causes an action potential to propagate in both directions from the stimulus site. In the van den Honert and Mortimer technique, a counter-current is applied on only one side of the electrode, which counter-current is of a sufficient magnitude to block propagation of the action potential in that direction from the excitation site. In the Fang and Mortimer technique, the counter-current is regulated to block large diameter nerve fibers while permitting the action potentials on the small diameter fibers to propagate past the end of the electrode. The action potentials propagating on the unblocked small diameter fibers cause flexing of the hind leg muscles with proportionately less force and less fatigue.
The present invention contemplates a new and improved technique for differentiating action potentials destined for different organs.